Resumes of the review papers and
discussions on terrestrial hydrological processes (chaps. 3, 4,
5, and 6) were made by Walther Manshard and Keith McNaughton and
on the papers on modelling and inputs to the atmosphere (chaps. 7
and 8) by Anne Henderson-Sellers. These resumes were discussed by
the workshop and some of the points raised are given in the
following sections.

Surface Processes

Deforestation is commonly less
extensive and complete than is generally imagined. Also,
hydrologists and climatologists are rarely conversant with the
social and economic reasons that govern this activity and with
the complete range of resultant vegetation types, their uses, or
the degrees of impoverishment that result. These two problems
could be remedied by investigations to give the historic
perspective of land use change in areas selected for their
hydrological and climatological significance.

The contrast between the very
large immediate changes in water yield in the first year
following deforestation and the subsequent, often marginal,
effects when a new vegetative cover has become established were
reviewed by Oyebande (chap. 3). These longer term effects on
water yield are rarely as great as 20% of the original value.
However, continuing interference with the regrowth of vegetation
could produce very large effects, and recovery might become
impossible.

In relation to afforestation or
reforestation, apart from work in Russia, most observations have
been on forest less than 20 years old. They therefore neither
corroborate nor contradict the observations in chapter 5 that
evaporation from the forest reaches a maximum at about 20 years.
One result from older Australian eucalypt forest suggests that
the Russian finding may apply to other areas. Another suggestion
is that forest water use is correlated with the density of the
tree stand. It is important to note that, increasingly, studies
are examining the underlying processes involved rather than being
simply observations of phenomena (chap. 3).

Although changes in the time
course of runoff after rainfall may have little relevance to the
effects of vegetation on climate, they can be an important result
of vegetation change. Vegetation change may lead to modifications
in water quality, soil erosion, and flooding all of great
significance to the local inhabitants.

On a small scale, some of the
effects of forest on precipitation may be explicable in terms of
the redistribution of rain or snow; this would cause little
effect at the 20 km x 20 km scale. There is a possibility that
recycling of precipitation could enable changes in vegetative
cover to cause changes in precipitation significant on a 500 km x
500 km scale. However, there are likely to be difficulties in
proving and quantifying this.

It is arguable how studies on the
climatological effects of afforestation or deforestation might
best be furthered by institutional and organizational means.
There is a great deal to be said for informal but productive
meetings of scientists exchanging information at small seminars
held in third world countries. Another approach might be to
establish a documentation centre. This would need a well-informed
staff and adequate financial backing to provide an efficient,
up-to-date service. Ideally, the centre would distribute
validated data, lists of all specialists and their work, and
authoritative reviews. It would also be able to give advice on
priorities, techniques, and software. There is much to be said
for such a centre to be integrated with a training or research
institute, or even with an international network of such
institutes.

Data needed for GCMs are
available from several independent sources; however, there is
sufficient disagreement between some data sets to materially
affect the simulations of the models. These discrepancies are not
confined to a single parameter: they apply, for instance, to
estimates of areas of vegetation types, areas of deforestation,
and magnitudes of carbon sinks. Clearly there is a need to
resolve these differences.

Some of the incompatibility
between data sets is related to the different scales of the sets.
That is, the number of classes of soil, vegetation, or land use
often increases with scale. Perceived spatial heterogeneity is
thus intimately linked with scale. If data are simply entered as
the value at specified grid points, they may well lead to errors
in the simulation. An improvement in this case would be to also
include information on the spatial pattern of the parameter.
There are indications that an appropriate pattern can be related
to drainage basins. Within a basin there is a degree of
orderliness among land surface parameters, while a considerable
amount of the heterogeneity is associated with distinctions
between basins. An initiative by the WMO is moving in the
direction of making greater use of this relationship. River
basins, as well as being generally well defined topographically
and governing the hydrology of water flow (see chap. 7), are also
being increasingly used as the basis for planning and
development. It seems entirely appropriate that ground surface
data used in GCMs should attempt to combine these grid and river
basin approaches. There are, of course, other problems of scale
(chap. 7) and problems due to heterogeneity in time, both
systematic and random, that need more attention if the outputs
from GCMs are to be improved.

Enough experience has been gained
from simulation experiments (chap. 8) to indicate that land
surface parameters can certainly affect the regional climate.
Thus vegetation change by man, provided it causes changes in
albedo and evaporation to approach the values inserted in the
models, could cause regional changes in climate. Admittedly, one
of the weakest parts of the models is the actual simulation of
precipitation, in comparison to other atmospheric processes and
states. However, the models consistently predict that if albedo
increases, evaporation descreases and precipitation markedly
decreases.

If the way forward is to refine
the models and conduct many more experiments with them, then
simulating earth surface changes closer to values possible with
society's current economy and technology is needed. This in turn
requires more accurate and detailed data collection for the whole
of the earth's surface. Morever, the reliability of the models
must be firmly established if planning and investment decisions
are to be based on them. This gives support to the three extant
proposals to collect mesoscale data that are relevant to regional
atmospheric circulation and the terrestrial inputs involved. It
seems desirable that initially a test site should be established
in the Brazilian Amazon. At least one of the proposals envisages
an extension to other regions at a later stage.

One proposal is outlined in
chapter 7. It involves a test of the parametrization in models of
forest evaporation, soil moisture, and runoff. The scale of a
pilot study would be 50 km x 50 km and would most appropriately
be based on the existing Brazilian/lnstitute of Hydrology, UK
observation station at the Duce Reserve, Manaus.

Another proposal (ALVIM,
"Isotope Aided Studies of the Effects of Changing Land Use
on the Ecology and Climate of the Brazilian Amazon") is for
the quantitative study of the water, nitrogen, carbon dioxide,
and other nutrient cycles. It also aims to identify the origin of
water-vapour producing precipitation on the Amazonian region and
to establish better water vapour circulation models.

The third study, the
"Diurnal Amazon Regional Climate Experiment," proposed
under the names of Molion and Dickinson, intends to relate
radiosonde measurements of climatic parameters to measurements of
rainfall and forest microclimate at surface sites. These combined
measurements would be related to global weather patterns and
together with satellite radiation measurements be used to test
and improve GCM and mesoscale models.

Hopefully, following further
development and critical tests of GCMs, within a decade the
scientific community will be in a position to predict with some
confidence what would be the climatic effect of any proposed
large-scale change in land use. This will enable both national
and international planners and investors to take into account the
effect on the regional or even global climate of their
activities.

With the objectives of
stimulating projects in the most significant areas of the
subjects considered by the workshop and publicizing the current
state of knowledge regarding the effects of vegetation change and
regional climate, the experts agreed on the following
conclusions.

The Description of Vegetation Change. Large-scale changes in vegetation
induced by man, of which deforestation is the most extreme
example, often result in site impoverishment (in terms of
productivity, fertility, flora, and fauna) and an alteration of
regional heat and water balances. Deforestation also results in
an increased input of CO2 to the atmosphere, though
not as large as that resulting from the burning of fossil fuels.

To understand the role of
vegetation change in these effects data are needed relevant not
only to the size of the area involved, about which there is
considerable disagreement, but also to the significant physical,
chemical, and biological parameters (perhaps remotely sensed and
monitored): for example, albedo, aerodynamic roughness, and
biomass. It may be possible to assess the relevant parameters
from an appropriate ecological description. There is a real need
to specify and quantify more precisely vegetation change in terms
of the regrowth of vegetation and/or subsequent land use. For
selected areas, the history of human interference and system
response need to be thoroughly investigated.

Hydrological Processes in Temperate and Tropical Regions. In the tropics more rapid hydrological
cycling occurs than in temperate regions, convective rainfall is
more important, as are the meteorological responses to surface
fluxes of heat and moisture. The net effect of these processes
during land changes and climatic anomalies may well be more
critical in tropical regions as atmospheric demand is high and
persistent. In addition, the most extensive land use changes are
currently occuring in these areas.

The Importance of Forest Succession and Age on Hydrology. With forest clearance there is an
immediate, short term increased discharge (of up to 450 mm per
year). Subsequent tree growth produces well-known and important
changes in surface hydrology. In relation to the detailed Russian
findings on the effects of forest age and succession on site
hydrology, it would be useful to obtain comparable data for other
regions and to establish the mechanisms involved. In the tropics,
rapid growth rates mean that any analogous effects would operate
over greatly reduced time-scales: for natural tropical forest
there may well be no effect of stand age due to the small-scale
heterogeneity of species and age in these forests.

Forests and Changes of Precipitation. Many apparent local variations in
precipitation due to forests over a few tens of kilometres at
most are essentially caused by redistribution of precipitation
and as such are edge effects. There is some evidence for about a
5% increase in precipitation due to aerodynamic roughness of
forests. Although the mechanisms of this need to be clarified, it
is probably due again to precipitation redistribution rather than
to an actual increase. In the tropics, modification by forests of
the surface hydrology is likely to have some effect on
precipitation but probably to a small degree. All other possible
changes of precipitation caused by forest are scale dependent.

Changes of vegetation cover on a
massive scale (e.g. a GCM grid scale) may result in precipitation
change on this scale in areas where recycling via evaporation is
important and the Charney mechanism may operate. The magnitude of
this change of precipitation would depend on the change in
vegetation and soil characteristics such as albedo, surface
roughness, interception capacity, infiltration rate, and water
storage capacity. Calculations predict that the conversion of a
large area of tropical forest to pasture might result in a
decrease in precipitation of about 200 mm per year. Feedback
processes involving surface albedo, vegetation development, and
soil and atmospheric moisture might amplify these changes.

The Future Role of Catchment Studies. Catchment studies will continue to be
needed as an integrated description of the performance of surface
hydrological systems. However, the uniqueness of each catchment
limits the predictive value of these investigations for other
catchments. They therefore need to be supplemented by shortterm
process studies that in time can be validated by the catchment
studies. Catchment studies in the tropics should be multiplied
and also extended into larger flat basins for comparison with
flatland micrometeorology.

The Use of Isotopes to Study the Recycling of
Precipitation. The use of
stable isotopes as an independent approach has promise for
assessing the recycling of water, especially in the tropics where
precipitation is largely of convective origin. The measurement
precision must be adequate and fractionation processes completely
understood. Detailed process and modelling studies of recycling
should be used to complement the isotope method.

Extrapolating Hydrological Results in Space and Time. The heterogeneity of important
hydrological parameters and processes make extrapolation
difficult. For instance, local soil moisture measurements cannot
be applied to the whole catchment due to the small-scale
variability of soil and our poor understanding of runoff
processes. Simplistic models for runoff and soil water
availability have to be adopted. Although widely used, these are
poor predictors of macro-catchment behaviour.

Ideally, appropriately scaled
models should be used for extrapolating to a larger scale. For
example, when proceeding from microscale to mesoscale, the model
should include boundary layer considerations defined from tower
and plot studies. Such models of the atmospheric boundary layer
processes are needed to integrate plot studies over larger
catchments and to interface surface processes to GCM models.
Larger catchments need to be studied due to the increased numbers
of interactions and feedbacks as the scale increases.

As most models work with short
time steps, integration over time is not generally a problem.
However, the model must be demonstrably valid for the time scale
of the simulation. A further complication is that vegetation
succession could alter surface properties over a long period.

The Importance and Design of Mesoscale Experiments. Mesoscale experiments are essential, on
the one hand to explore the hypotheses of macro-hydrology at the
scale of GCMs and, on the other, to test mesoscale data
synthesized from microscale and watershed data. Mesoscale
experiments must include studies of horizontal integration in the
lower atmosphere.

The pilot experiment proposed
under the World Climate Research Programme is an example of a
mesoscale experiment on an area of 50 km by 50 km. Land surface
fluxes will be estimated by a number of independent methods.
Radiation, precipitation, evaporation, runoff, and soil moisture
will be measured. During the period of transition from soil
saturation to large soil moisture deficits, about a hundred hours
of measurement of the boundary layer will be made from an
aircraft.

The Importance of Increased Carbon Dioxide in the
Atmosphere on the Global Hydrologic Cycle. Anthropogenic increase of atmospheric
carbon dioxide and the predicted global warming may have
significant effects on forests and thus on the hydrologic cycle.
Recent GCM simulations of regional climatic response to
atmospheric carbon dioxide increases, though preliminary,
indicate that marked changes, both negative and positive, might
be expected for precipitation and evapotranspiration for certain
regions of the earth. Improved simulations should indicate the
regions where this effect of carbon dioxide change is most likely
to produce detectable hydrologic changes. It is possible that in
some areas the effects of carbon dioxide increase will exceed the
direct hydrological impact of land use changes.

Under laboratory conditions
raised carbon dioxide levels increase carbon assimilation by
plants and decrease transpiration, that is, increase the water
use efficiency of at least some species. However, the increased
temperatures could be accompanied by increasing respiration and
stress. The potential of these interactions on the hydrology of
areas under natural and crop species is difficult to assess.

Although land use changes may
significantly alter global biomass and must be considered in any
interpretation of the partitioning of carbon from fossil fuel
between the major carbon sinks, the past, present, and future
losses of biospheric carbon are unlikely to have a major effect
on atmospheric carbon dioxide fifty to a hundred years hence.

Global Climatic Effects of Vegetation Change. Vegetation change may contribute to
changes in the global mean temperature in a number of ways:

through contributing
slightly to increases in atmospheric carbon dioxide and
hence to any carbon dioxide induced global warming;

by changes in surface albedo
resulting in a temperature change; however, a simulation
model of a loss of tropical forest equivalent to 3% of
the earth's surface showed that the effect was unlikely
to be significant compared to the natural variability of
the temperature;

by changes in interception
and runoff modifying evaporation, surface temperature,
and atmospheric moisture, and ultimately the radiation
balance: these effects cannot be quantified on the
available evidence.

Other impacts on climate have not
been assessed, but it should be noted that changes in the
distribution of tropical heat sources have been shown by other
modelling experiments (e.g. sea temperature anomaly) to affect
the global circulation at all latitudes. However, from the
current evidence it would seem that changes in vegetation are
more likely to have important effects on climate at the regional
scale.